Electromagnetic heating system which includes an automatic shielding mechanism and method for its operation

ABSTRACT

A tray inserted into an electromagnetic oven positions an energy shield to interfit with elements on the tray to isolate selected items upon the tray from the electromagnetic energy and operates interlock devices to activate the microwave energy source.

ll'nited States Patent [1 1 Moore et al.

ELECTROMAGNETIC HEATING SYSTEM 1 Dec. 10, 1974 3,219,460 11/1965 Brown 219/1055 E WHICH INCLUDES AN AUTOMATIC 3,547,661 12/1970 Stevenson SHIELDING MECHANISM AND METHOD 1615/13 10/197 3,657,508 4/1972 Studnick 219/85 FOR ITS OPERATION [75] Inventors: Donald G. Moore, Glencoe, 111.;

William E. Layer Loui ville, Ky, Primary Examiner-Bruce A. Reynolds a F a a a [73] Assignee: filhemetron Corporation, Chicago, gf g g z Mason Kolehmamen [22] Filed: July 18, 1973 52] UTSTGT .3:11.11..jfii: 219/1055 A tray inserted into an electromagnetic oven positions [51] Jim. 1105b 9/06 an energy shield to interfit with elements on the tray [58] Field of Search 219/1055, 85; 99/447; to isolate selected items upon the tray from theelec r 126/390 tromagnetic energy and operates interlock devices to l activate the microwave energy source. [56] References Cited 1 1 v UNITED STATES PATENTS 37 Claims, 8 Drawing Figures 2,600,566 6/1952 Moffett 219/1055 E HEATlNG EQUALIZING SERVE PATEHI xmmQM I SHEH 1 W 3 0.0" HEATING EQUALlZING SERVE ELECTROMAGNETIC HEATING SYSTEM WHICH INCLUDES AN AUTOMATIC SI-IIELDING MECHANISM AND METHOD FOR ITS OPERATION CROSS REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION The present invention relates to electromagnetic heating systems and, more particularly, to systems which selectively heat various elements of a load'such as a complete meal or the like in a single operation. In particular, the present invention provides a combination for moving a shield over certain food items or the like by inserting a tray conveying those items into an oven and to a method for its operation.

In institutions, such as hospitals, it is desirable to provide a means whereby a complete meal may be brought up to serving temperature in a single, quick operation. If this can be done, the meals may be prepared in an assembly-line fashion on weekdays and during normal working hours, and then they may be kept refrigerated until the time when they are to be served.

A typical meal normally includes some food items which require heating before they may be served and other food items which are preferably served cold. For example, the salad and dessert portions of a meal are preferably served cold. The main entree of a meal is preferably served hot. If a meal includes coffee or tea, the coffee or tea is preferably served very hot.

A conventional oven ordinarily supplies heat energy to every portion of the meal. In an electromagnetic oven, on the other hand, any food item that is completely enclosed inan electrically conductive wrapper or shield is not heated. It is therefore possible to heat acomplete meal in a microwave oven without heating, for example, a dessert such as ice .cream that is wrapped in aluminum foil. However, wrapping all itmes such as desserts, salads, and beverages such as milk in foil adds considerably to the cost of preparing a meal. Difficulties may also arise any time a shielded item is placed into an electromagnetic oven because the shield distorts the electromagnetic field within the oven. Arcing can also result, especially if the shielding has any sharp edges or is positioned adjacent a conductive wall of the oven chamber. I

Placing a protective box by hand on each tray over items to be kept cool is effective but laborious and can result in errors of misplacement or completeomission.

There is a need, therefore, for an improved apparatus and method to provide shielding for selected items to be heated in a microwave oven together with an interlock mechanism ,to operate the'oven only when the shield is properly in place.

.. BRIEF SUMMARY O THE INVENTION An objectof this invention is the automatic shielding of some of the objects in an oven from heating'in the oven.

A primary object of the present invention is to provide a mechanism which automatically shields some of equivalent means. Preferably, the tray is supported by the items on a tray from electromagnetic energy when the tray is inserted into an electromagnetic oven.

A further object of the invention is provision of a position-sensing mechanism arranged to respond to the final location of the shield and tray within the oven.

A still further object of the invention is provision of a mechanism which gives forth an audible warning and disables the oven until the problem is corrected when the shield is not properly seated against the tray.

A further object of the invention is provision of a method for heating objects to a non-uniform temperature in accord with a controlled energy distribution and object placement in the oven.

Other objects, features and advantages of the invention are apparent in the detailed description which follows. The features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification.

. These and other objects of the invention are achieved by provision of an electromagnetic oven system for heating materials carried on a tray comprising an oven cavity having conductive walls and an open end for receiving trays inserted therein, a door to open and close said open end, means for directing microwave energy into said cavity, a conductive shield mounted for movement in said cavity between an open tray-receiving position and a closed position shielding a portion of a tray inserted in said cavity from microwave energy therein, and actuator means responsive to the insertion of a tray into said cavity for moving said shield from said open to said shielding position.

The invention includes the method of heating objects in a'cavity adapted to receive heating amounts of microwave energy comprising the steps of locating the objects in a predetermined array on a tray, lowering a shield over the objects in response to the insertion of the tray in the cavity and heating those objects not within the shield.-

In a preferred embodiment the method includes the steps of securing the shield and activating a heating circuit in response to closing of the door'to the cavity.

The invention also includes the steps of grouping to gether items which are to be heated in a first region called a heating zone, and assembling items which are not to be heated in a second region called a protected zone. In the preferred embodiment of the invention, the items are arranged on a tray whereon the heating zone is an L-shaped region which partially-surrounds the protected zone.

When a tray is inserted into the oven, the mechanism lowers a shield within the oven to cover all items located in the protected zone of the tray; closing the oven door secures the shield in place and activates an enabling mechanism in the heating control circuits. This shield may comprise a box-like structure that .is hingedly mounted within the oven and that is drawn down into position either by the motion of the tray into theoven, by the closure of the oven door, or by other a conductive shelf or bottom wall of the oven which also serves as a bottom wall for the box-like shield. The back and side conductive walls of the oven may also cooperate with the shield, if desired.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, frequent references will be made to the drawings wherein:

FIG. 1 is a front perspective view of a microwave oven constructed in accordance with the teachings of the invention and showing the shield in a raised positron;

FIG. 2 is a perspective view of the shield which is shown hingedly mounted within the oven of FIG. 1 and which is automatically brought into position to shield certain food items resting upon the tray shown in FIG.

FIG. 2A is a front, elevational view of the shield shown in FIG. 2;

FIG. 3 is an isometric view of a tray for use in the oven shown in FIG; 1, additionally adapted to cooperate with the shield shown in FIG. 2, and also carrying a ferrite oven control element;

FIGS. 4 and 5 are sectional views of the oven shown in FIG. 1 taken along the lines 44 in FIG. 1 to illustrate the manner whereby the shield shown in FIG. 2 is brought down into place to protect items upon a tray that is inserted into the oven;

FIG. 6 is a front sectional view taken along the lines 6-6 in FIG.-5 to illustrate engagement of the shield shown in FIG. 2 with upstanding flanges upon a tray and the oven bottom to minimize the amount of electromagnetic radiation which reaches the shielded items placed upon the tray; and

FIG. 7 is a right-hand sectional view of the oven shown in FIG. 1 taken along the section lines 7+7 to show the mechanical mechanism which is actuated when the oven door is closed and which secures the shield into protecting-position within the oven.

DESCRIPTION OF THE PREFERRED EMBODIMENT Thepreferred embodiment of the invention is a modified version of a conventional microwave oven. A suitable oven is a system 70/80 MenuMaster oven sold by the Atherton Division of Litton Industries, Inc., Minneapolis, Minnesota, but other conventional microwave ovens may also be used in constructing the invention.

Referring now to the drawings, therein is illustrated an oven 100 having electrically conductive walls to accept food items distributed over a tray as described below. The back wall 101 of the oven has sockets 102 and 104 between which extends a movable tongue 106. The floor 108 of the oven 100 is constructed from an electrically conductive non-magnetic material. The oven floor and back wall cooperate with the shield 200 to form a substantially closed, electrically conductive enclosure.

An angle 109 secured to the floor and extending up and across the back wall 101 acts as a guide for the tray and aids to seal the shield 200 shown supported by the tongue 106 which is raised byspring biasing means. Dotted lines 120 indicate elements of the control mechanism which cooperate with a sensor on the tray to control oven operation. Indicator lamps -1 16 118, and 119 disclose the" condition of the oven operating sequence. Hook 402 assists in supporting the door 112 and also operates elements of the interlock mechanism as discussed further below.

includes rearwardly extending fingers 202 and 204 which engage corresponding sockets 102 and 104. Bracket 208 secures the shield to tongue 106. Leaf spring 206 depends within the shield and, in part, Iowers the shield when a tray is in the oven.

The shield 200 has its lower edge 212 coated with an electrical insulator 210, e.g., Teflon tape to prevent arcs from forming between the shield 200 and the angle 109, upstanding flanges on the tray 310 and 312 or the strip 318 (FIG. 3). The shield 200 is so arranged within the oven 100 that when lowered into position over a tray, it creates a region shielded from the electromagnetic radiation that .is developed within the oven 100.

Portions of the corners 214 and 216 are provided with contours which complement the edges of the tray against which it is located in the protecting position. In alternate embodiments, a plurality of leaf springs of similar configuration to spring 206 may assist in lowering the shield. The bracket 208 may be formed integrally with the top of the shield.

The tray 300 shown in a preferred embodiment in FIG. 3, has an upper side 301 and edges 301A. An L- shaped conductive strip 318 on its upper side 301 supports a pair of conductive, upstanding flanges 310 and 312 which in part define zones 302, 304, and 306 within which load items may be placed and provide coupling to the cavity floor for microwave energy. Since the flanges 310 and 312 engage the shield 200 on opposite surfaces, it is necessary to have the two flanges 310 and 312 separated from one another by a notch 322.

The flanges 310 and 312 in the L-shaped strip 318 have dimensions, including radii, which are selectedto 'reduce field concentrations and to effect an electromagnetic energy seal with the floor of the oven and -with the front and side wall of the shield 200. The body of the tray is constructed from polyester fiber glass which has excellent mechanical strength but only fair dielectric properties. Barrier 319 serves to direct load items to locations where they will not interfere with the lowering of the shield. In a preferred embodiment, a ferrite sensor 308 is cemented or otherwise attached to each of the trays 300 and is part of the oven control system. A cover 314 overlies the sensor 308 to provide mechanical protection in addition to regulating the amount of microwave energy which reaches the sensor.

As is illustrated in FIGS. 4-6, tongue 106 extends outwardly from the oven back wall and engages bracket 208 upon the top of the shield 200. The tongue is rigidly attached to a shaft 416 (FIGS. 4, 5 and 7) which runs the length of the back wall 101 of the oven 100 and which is mounted at its two ends by means not I shown which permit the shaft 416 to rotate. The shaft 416 is also rigidly attached to a lever 412 (FIGS. 4, 5, and 7) which extends forwardly along the right-hand wall of the oven 100 (see FIG. 4). A first spring 414 normally biases the lever 412 to apply an angular FIG. 2 depicts the shield 200 which is shown pivotally torque to the shaft 416 which tends to rotate the tongue 1 06 upwards within the oven. A stop (not shown) limits the upward motion of the tongue 106. When the rearwardly-extending fingers 202 and 204 of the shield 200 are inserted into sockets 102 and 104 in the back of the oven, the bracket 208 is slipped over the tongue 106 so that the angular positioning of the shield 200 within the oven is determined by the position of the tongue 106.

The biasapplied to the lever 412 by the spring 414 causes the tongue 106 to hold the shield 200 normally suspended in air when no tray is present.

Other features of the mechanism are illustrated in FIG. 7. A crank 406 mounted to rotate about its support 407 is connected by chain 408 to a hook 402 mounted on the door 112. Tension spring 420 opposes the action of the chain. The end 409 of the crank is connected by spring 410 to the lever 412. A SPDT switch 418 is' mounted for actuation by the lever 412 and a normally open switch 422 is located for actuation by the end 409 of the crank 406. The normally open contacts of the switch 418 are connected into the oven control circuit and the normally closed contacts are in an alarm circuit with contacts of the switch 422.

In operation of an embodiment of the invention, food itmes are arranged on a tray so that items not to be heated are all placed within the protected zone 303; food items to be heated are placed within one of the heating zones 302, 304 and306.

When the tray 300.is inserted into the oven 100, the leading edge 320 of the tray engages the leaf spring 206 and draws the sheild 200 down substantially over the region 303 on the tray.

The tensionof the spring 414 is relatively weak and is easily overcome when a tray 300 presses against the leaf spring 206 that is attached to the shield 200. Since the tray is moving rearwardly at the same time that the leading lower edge 212 of the shield 200. is moving through a rearwardly-directed are, both the tray 300 and the lower edge 212 are simultaneously moving in the same direction, so that the edge'212 moves almost straight down relative to the tray 300 and does not strike food items 326 positioned toward the front of the tray. If the shield 200 were brought into position only by closure of the oven door 112 and not by motion of the tray 300, then therearwardly-directed arcthrough which the leading edge 212 of the sheild 200 swings would sweep over the heating region of the tray, possibly striking against taller food items positioned in that region. The action of the' leaf spring 206 eliminates this problem and allows larger items to be placed upon the tray than would otherwise be possible. If the leaf spring 206 is omitted from the shield 200, then the shield 200 is brought downwards over the tray 300 by the tongue 106 which is actuated by the closure of the oven door 112.

If a food item does happen to get under the leading edge 212 of theshield 200, the leaf spring 206 flexes and does not permit the shield 200 or the food item to be damages. In a preferred embodiment, the relatively weak leaf spring 206 need not completely seat the shield 200 upon the tray 300 as .a separate dooractuated lever mechanism is provided for that purpose.

vAs shown in FIG. 7, while the door 112 of the oven is open, a hook 402 attachedvto the door 112 pulls on a chain 408 to stretch a spring 420 and rotate the crank 406 so that the crank 406 applies no tension to the spring 410. The spring 410 thus has no effect upon the lever 412 when the oven door 112 is open.

When theoven door 112 is closed, the chain 408 crank 406 in a'clockwise direction to'the limit of its travel.,The' spring 410 then exerts a strong effort upon the lever 412. This effort develops a torque which is transmitted by the shaft 4l6to the-tongue 106 and thence to the shield 200. This torque seats the sheild 200 firmly upon the tray300 within the oven 100.

The normally open contacts of a position-sensing SPDT electrical switch 418, mounted adjacent the lever 412, are closed by the lever 412 only if the lever 412 is rotated into a position corresponding to a properly-seated shield 200. If the shield 200 is prevented from seating properly, then the lever 412 does not swing far enough to actuate the switch 418. If the shield 200 is not in place when the oven door is closed, then the lever 412 is free to rotate beyond the position at which it actuates the switch 418 and again the contacts do not close. Unless the lever 412 closes the contacts of the switch 418, the heating circuits of the oven do not function. If the switch 418 is not actuated when the oven door is closed, the normally closed contacts en-, able switch 422 to give an audible warning to the oven operator that something is amiss.

After the oven door 112 is closed, heating of the food items within the zones 302, 304 and 306 commences. The interior of the oven is flooded with microwave energy at a frequency of 2,450 megacycles generated by a conventional magnetron microwave energy source. During the heating process, HEATING lamps 116 on the front of the oven are illuminated to signal that microwave energy is,"in fact, being supplied to the oven. When the heating process if finished, a SERVE lamp 1 19 automatically comes on to signal that the tray may be removed from the oven. At this time, the oven door 112 is opened by grasping and pulling down on the handle '114 and the tray 300 is removed from the oven 100. Engagement of the corner 216 of the shield with the rear rim of the tray as the tray is withdrawn aids in elevating the shield to its raised position. All items within the protected zone 303 are still cold, since they were shielded by the shield 200; other items upon the tray have been heated.

While the embodiment illustrated herein was deheat capacity.

, goes limp and permits the spring 420 to rotate the.

rials carried on a tray comprising an oven cavity having electrically conductive walls and an open end for receiving trays inserted therein, a door to open and close .said open end, means for directing microwave energy 'into said cavity, an electrically conductive shield mounted'for movement in said cavity between an open tray receiving position and a closed position, means including said shield when in said closed position and cooperating with the bottom wall of said cavity to provide substantial shielding of a portion of a tray inserted in said cavity from microwave energy therein, and actuator means for moving said shield from said open to said closed position.

2. The oven system of claim 1 wherein said actuator means includes linkage means interconnecting said door and said shield and switch means responsive to said linkage whereby said switch assumes a heating enabling position when a tray is properly shielded in said oven.

3. The oven system of claim 1 including means for detachably and pivotally interconnecting said shield and a rear wall of said cavity, said actuator means including an element detachably engaging said shield for operatively interconnecting the same with said linkage means.

4. The oven system of claim 1 wherein said actuator means includes a tongue extending into said cavity from said rear wall and having a free outer end, said shield including attachment means for receiving the free end of said tongue.

5. The combination of claim 1 wherein said shield includes opposed side walls each having a lower edge, a strip in conductive relation with the bottom wall of said cavity and adapted to interfit in side by side shielding relation with a lower edge of at least one of said side walls when said shield is in saidclosed position.

6. The combination of claim 5, which includes insulating means positioned between said strip and said lower edge of said shield when said shield is in said closed position, thereby to prevent arcing between said strip and said lower edge of said shield.

7. The combination of claim 1 including on said tray conductive means defining upstanding flanges adapted to interfit in side by side shielding relation with lower edge portions of the side and front walls of said shield when the latter is in said closed position.

8. The combination of claim 7, which includes insulating means positioned between said flanges and said lower edge portions when said shield is in said closed position.

9. The oven system of claim 1 in combinationwith said tray, said oven cavity having a conductive bottom wall for supporting said tray and a rear wall adjacent one edge thereof, said tray including a conductive strip on the upper surface thereof outlining at least a part of said portion to be shielded, and said shield having at least a front wall with a lower edge positioned adjacent said conductive strip when the shield is in said closed position.

10. The oven of claim 1 wherein said actuator means includes a depending spring within said shield for engaging the edge of a tray as it is inserted in said oven cavity from said open end for moving said shield from said open toward said closed position.

, 11. The oven of claim 10 wherein said spring is positioned to be engaged by said tray before said tray is fully inserted in said cavity whereby continued tray insertion and pivotal movement of said shield toward said closed position proceed simultaneously.

12. The oven .of claim 10 wherein said actuator means includes second linkage means interconnecting said door and said shield for biasing said shield'forcefully against an inserted tray in response to closing of said door.

13. The method of heatingiobjects in a cavity adapted to receive microwave energy comprisingthe steps of locating the objects in a predetermined array on a tray, lowering a shield over the objects in response. to insertion of the tray in the cavity, and heating those objects not within the shield.

14. The method of claim 13 including the step of securing said shield in position over a tray in said cavity, and activating a heating circuit in response to closing a door to the cavity.

15. An electromagnetic oven system for heating selected materials on a tray comprising an oven cavity having an opening for receiving trays, a door to open and close said opening, means defining a horizontal conductive tray support in said cavity, means for supplying microwave energy to said cavity, an electrically conductive shield mounted for movement in said cavity between an open tray receiving position and a closed position, means including said shield when in said closed position and cooperating with said tray support to provide substantial total shielding of a portion of a tray inserted in said cavity from microwave energy therein, and actuator means for moving said shield from said open to said closed position.

16. The combination of claim 15, wherein said tray is made of insulating material.

17. The combination of claim 16, wherein there is provided means for preventing the bottom edge of said shield from producing dielectric breakdown of the material of said tray in the vicinity of said bottom edge when said shield is in said closed position.

18. The combination of claim 15, wherein said tray is made of plastic material, and conductive means on said tray and adapted to cooperate with the bottom edge of said shield when in said closed position to distribute the microwave field within the material of said tray in the vicinity of said bottom edge.

19. The combination of claim 18, wherein said conductive means includes a thin conductive strip secured to said tray and positioned to be in the vicinity of the bottom edge of said shield when said shield is in said closed position.

20. The combination of claim 19, wherein said conductive strip is relatively wide as compared to said bottom edge of said shield, thereby to reduce field concentration within the material of said tray.

21. The combination of claim 15, wherein said tray is made of insulating material and is provided with a relatively wide conductive strip on the upper surface thereof in the vicinity of the bottom edge of said shield when said shield is in said closed position, thereby to prevent microwave energy from entering the area of said tray covered by said shield.

22. The combination of claim 15, which includes means for moving said shield from said open position to said closed position as said tray is inserted into said cavity and in such manner that the forward edge of said shield moves substantially vertically in relation to said tray as it is inserted. 1

23. The combination of claim 15, which includes a member mounted on said tray in a position somewhat ahead of the forward edge of said shield when said shield is in said closed position, whereby said member acts as a barrier to prevent food items from being positioned in an area where they will be struck .by said shield as it is moved to said closed position.

24. The oven system of claim 15, wherein said actuator means is responsive to insertion of a tray into said cavity for moving said movable shield portion from said open to said closed position.

25. The combination of claim 15, which includes means for preventing the application of microwave energy to said cavity in the event said shield is not properly positioned relative to a tray on said conductive tray support in said cavity.

26. The combination of claim 15, wherein said shield is detachably mounted in said cavity, and means for preventing the application'of microwave energy to said cavity in the event a tray has been inserted in said cavity and said door closed without said shield also being mounted in said cavity.

27. An electromagnetic oven system for heating selected materials on a tray comprising, an oven cavity having an opening for receiving trays, means for supplying microwave energy to said cavity, and shielding means at least a portion of which is attached to and positioned within said cavity for substantially totally shielding a portion of a tray positioned in said cavity from microwave energy therein.

28. The combination of claim 27, which includes actuator means for moving a portion of said shielding means from an open tray-receiving position to a closed shielding position.

29. The combination of claim 28, wherein said movable shield portion includes conductive side and front walls, the bottom edges of which are positionedover said tray when saidshield portion is in said closed position, thereby to define said totally shielded portion of said tray.

30. The combination of claim 28, wherein said shielding means includes .a rear wall of said cavity which cooperates with said movable shield portion to define said totally shielded'portion of said tray.

31. The combination of claim 28, wherein said shielding means includes a conductive tray support within said cavity which cooperates with said movable shield portion to define said totally shielded portion of said tray.

32. The combination of claim 28, wherein said movable shield portion is pivotally mounted for movement within said cavity.

33. The combination of claim 32, wherein the tray is provided with a rim portion, and means on said movportion is detachably mounted in said cavity, and alarm means for indicating when a tray has been inserted into said cavity without said shield portion being alsopositioned within said cavity.

36. An electromagnetic oven system for heating selected materials on a tray comprising, an oven cavity having an opening for receiving trays, means for supplying microwave energy to said cavity, means for positioning a tray at a predetermined location within said cavity, and shieldingmeans at least a portion of which remains in said cavity while a plurality of trays are successively positioned within said cavity for substantially completely shielding a portion of a tray positioned at said predetermined location in said cavity from microwave energy therein.

37. The method of selectively heating objects in a cavity adapted to receive microwave energy comprising the steps of, locating the objects in a predetermined array on a tray with at least one of said objects in a predetermined area of said tray which is unshielded when outside said cavity, inserting said tray with said objects located thereon into a predetermined area of said cavity, substantially completely shielding said area of a tray when located in said predetermined area of said cavity against microwave energy therein, and supplying microwave energy to said cavity to heat those objects outside said shielded portion of said tray. 

1. An electromagnetic oven system for heating materials carried on a tray comprising an oven cavity having electrically conductive walls and an open end for receiving trays inserted therein, a door to open and close said open end, means for directing microwave energy into said cavity, an electrically conductive shield mounted for movement in said cavity between an open tray receiving position and a closed position, means including said shield when in said closed position and cooperating with the bottom wall of said cavity to provide substantial shielding of a portion of a tray inserted in said cavity from microwave energy therein, and actuator means for moving said shield from said open to said closed position.
 2. The oven system of claim 1 wherein said actuator means includes linkage means interconnecting said door and said shield and switch means responsive to said linkage whereby said switch assumes a heating enabling position when a tray is properly shielded in said oven.
 3. The oven system of claim 1 including means for detachably and pivotally interconnecting said shield and a rear wall of said cavity, said actuator means including an element detachably engaging said shield for operatively interconnecting the same with said linkage means.
 4. The oven system of claim 1 wherein said actuator means includes a tongue extending into said cavity from said rear wall and having a free outer end, said shield including attachment means for receiving the free end of said tongue.
 5. The combination of claim 1 wherein said shield includes opposed side walls each having a lower edge, a strip in conductive relation with the bottom wall of said cavity and adapted to interfit in side by side shielding relation with a lower edge of at least one of said side walls when said shield is in said closed position.
 6. The combination of claim 5, which includes insulating means positioned between said strip and said lower edge of said shield when said shield is in said closed position, thereby to prevent arcing between said strip and said lower edge of said shield.
 7. The combination of claim 1 including on said tray conductive means defining upstanding flanges adapted to interfit in side by side shielding relation with lower edge portions of the side and front walls of said shield when the latter is in said closed position.
 8. The combination of claim 7, which includes insulating means positioned between said flanges and said lower edge portions when said shield is in said closed position.
 9. The oven system of claim 1 in combination with said tray, said oven cavity having a conductive bottom wall for supporting said tray and a rear wall adjacent one edge thereof, said tray including a conductive strip on the upper surface thereof outlining at least a part of said portion to be shielded, and said shield having at least a front wall with a lower edge positioned adjacent said conductive strip when the shield is in said closed position.
 10. The oven of claim 1 wherein said actuator means includes a depending spring within said shield for engaging the edge of a tray as it is inserted in said oven cavity from said open end for moving said shield from said open toward said closed position.
 11. The oven of claim 10 wherein said spring is positioned to be engaged by said tray before said tray is fully inserted in said cavity whereby continued tray insertion and pivotal movement of said shield toward said closed position proceed simultaneously.
 12. The oven of claim 10 wherein said actuator means includes second linkage means interconnecting said door and said shield for biasing said shield forcefully against an inserted tray in response to closing of said door.
 13. The method of heating objects in a cavity adapted to receive microwave energy comprising the steps of locating the objects in a predetermined array on a tray, lowering a shield over the objects in response to insertion of the tray in the cavity, and heating those objects not within the shield.
 14. The method of claim 13 including the step of securing said shield in position over a tray in said cavity, and activating a heating circuit in response to closing a door to the cavity.
 15. An electromagnetic oven system for heating selected materials on a tray comprising an oven cavity having an opening for receiving trays, a door to open and close said opening, means defining a horizontal conductive tray support in said cavity, means for supplying microwave energy to said cavity, an electrically conductive shield mounted for movement in said cavity between an open tray receiving position and a closed position, means including said shield when in said closed position and cooperating with said tray support to provide substantial total shielding of a portion of a tray inserted in said cavity from microwave energy therein, and actuator means for moving said shield from said open to said closed position.
 16. The combination of claim 15, wherein said tray is made of insulating material.
 17. The combination of claim 16, wherein there is provided means for preventing the bottom edge of said shield from producing dielectric breakdown of the material of said tray in the vicinity of said bottom edge when said shield is in said closed position.
 18. The combination of claim 15, wherein said tray is made of plastic material, and conductive means on said tray and adapted to cooperate with the bottom edge of said shield when in said closed position to distribute the microwave field within the material of said tray in the vicinity of said bottom edge.
 19. The combination of claim 18, wherein said conductive means includes a thin conductive strip secured to said tray and positioned to be in the vicinity of the bottom edge of said shield when said shield is in said closed position.
 20. The combination of claim 19, wherein said conductive strip is relatively wide as compared to said bottom edge of said shield, thereby to reduce field concentration within the material of said tray.
 21. The combination of claim 15, wherein said tray is made of insulating material and is provided with a relatively wide conductive strip on the upper surface thereof in the vicinity of the bottom edge of said shield when said shield is in said closed position, thereby to prevent microwave energy from entering the area of said tray covered by said shield.
 22. The combination of claim 15, which includes means for moving said shield from said open position to said closed position as said tray is inserted into said cavity and in such manner that the forward edge of said shield moves substantially vertically in relation to said tray as it is inserted.
 23. The combination of claim 15, which includes a member mounted on said tray in a position somewhat ahead of the forward edge of said shield when said shield is in said closed position, whereby said member acts as a barrier to prevent food items from being positioned in an area where they will be struck by said shield as it is moved to said closed position.
 24. The oven system of claim 15, wherein said actuator means is responsive to insertion of a tray into said cavity for moving said movable shield portion from said open to said closed position.
 25. The combination of claim 15, which includes means for preventing the application of microwave energy to said cavity in the event said shield is not properly positioned relative to a tray on said conductive tray support in said cavity.
 26. The combination of claim 15, wherein said shield is detachably mounted in said cavity, and means for preventing the application of microwave energy to said cavity in the event a tray has been inserted in said cavity and said door closed without said shield also being mounted in said cavity.
 27. An electromagnetic oven system for heating selected mAterials on a tray comprising, an oven cavity having an opening for receiving trays, means for supplying microwave energy to said cavity, and shielding means at least a portion of which is attached to and positioned within said cavity for substantially totally shielding a portion of a tray positioned in said cavity from microwave energy therein.
 28. The combination of claim 27, which includes actuator means for moving a portion of said shielding means from an open tray-receiving position to a closed shielding position.
 29. The combination of claim 28, wherein said movable shield portion includes conductive side and front walls, the bottom edges of which are positioned over said tray when said shield portion is in said closed position, thereby to define said totally shielded portion of said tray.
 30. The combination of claim 28, wherein said shielding means includes a rear wall of said cavity which cooperates with said movable shield portion to define said totally shielded portion of said tray.
 31. The combination of claim 28, wherein said shielding means includes a conductive tray support within said cavity which cooperates with said movable shield portion to define said totally shielded portion of said tray.
 32. The combination of claim 28, wherein said movable shield portion is pivotally mounted for movement within said cavity.
 33. The combination of claim 32, wherein the tray is provided with a rim portion, and means on said movable shield portion and adapted to be engaged by said rim portion as the tray is removed from the oven to facilitate pivotal movement of said shield portion to said open position.
 34. The combination of claim 28, which includes alarm means for indicating when said movable shield portion is not properly positioned relative to a tray which has been positioned in said cavity.
 35. The combination of claim 27, wherein said shield portion is detachably mounted in said cavity, and alarm means for indicating when a tray has been inserted into said cavity without said shield portion being also positioned within said cavity.
 36. An electromagnetic oven system for heating selected materials on a tray comprising, an oven cavity having an opening for receiving trays, means for supplying microwave energy to said cavity, means for positioning a tray at a predetermined location within said cavity, and shielding means at least a portion of which remains in said cavity while a plurality of trays are successively positioned within said cavity for substantially completely shielding a portion of a tray positioned at said predetermined location in said cavity from microwave energy therein.
 37. The method of selectively heating objects in a cavity adapted to receive microwave energy comprising the steps of, locating the objects in a predetermined array on a tray with at least one of said objects in a predetermined area of said tray which is unshielded when outside said cavity, inserting said tray with said objects located thereon into a predetermined area of said cavity, substantially completely shielding said area of a tray when located in said predetermined area of said cavity against microwave energy therein, and supplying microwave energy to said cavity to heat those objects outside said shielded portion of said tray. 